Anti-lock braking system having pulsed pressure re-application

ABSTRACT

An anti-skid braking system for wheeled vehicles having fluid actuated brakes associated with the vehicle wheels, has speed sensors associated with the vehicle wheels, a control device responsive to speed signals from the speed sensors to actuate a pressure dump device to periodically release the fluid pressure applied to the brake of any wheel which is determined to be about to lock and to later re-apply the actuating pressure to that brake when the tendency of that wheel to lock has reduced, pulsing of the pressure dump device at intervals during the pressure re-apply phase to cause a repeated interruption to the re-application of pressure to that brake. The pulse timing of the pulsing of the pressure dump device is controlled by the occurrence of wheel angular deceleration being determined to be equal to or in excess of a predetermined threshold value, the magnitude of which is dependent upon events from the previous anti-lock braking cycle.

The present invention relates to anti-lock/anti-skid braking systems(ABS) for vehicles.

Anti-skid braking systems for vehicles comprise speed sensors on each ofthe wheels of the vehicle and a control device responsive to speedsignals from the speed sensors to periodically reduce the pressure ofactuating fluid to the brake of a wheel which is about to lock in aseries of on/off cycles, thereby avoiding or reducing skidding of thewheel.

The control device seeks to combine sensitive control, efficient brakingand effective steerability/stability by optimizing the proportion ofeach cycle in which the wheel is operating close to the tyre's maximumadhesion. As is well known, the magnitude of this braking force, whenexpressed as a function of the tyre's slip, is characterised by a curvewith an asymmetric hump (see curve CB--FIG. 2). The balance oflongitudinal and lateral forces which exist at the peak of this curveusually represent an optimum compromise for normal road-going vehicles.

Once the tyre reaches the negatively sloping region of the curve,braking effort begins to diminish--sometimes sharply--and lateraladhesion (curve CS--FIG. 2), which reduces progressively with increasingslip, becomes unable to maintain satisfactory steering or stability. Ifthe wheel strays too far into this region before corrective action istaken, then the control will become coarse, with large angulardecelerations and accelerations of the wheel, high slip levels andconsequent large-amplitude pressure cycling. To passengers, this will beapparent as a jerky, vibratory sensation.

Efficient braking requires that the wheel should spend as much time aspossible at, or very close to, the slip level which corresponds to thepeak braking force. This means that the pressure rise rate has to berelatively flat, so that the time taken for the wheel to pass throughthe region of maximum braking force is prolonged. Control refinementwill also benefit through a reduction in "pressure overshoot", i.e. thedegree to which the brake pressure exceeds the ideal pressure during thetime needed by the system to detect the impending skid and initiatecorrective action.

Effective steering/stability is best assured by allowing the wheel tospend part of each cycle in the positively-sloping, i.e. underbrakedregion, of the curve. This occurs naturally as a consequence of pressureundershoot during the pressure dump phase, but needs to be controlled byproviding rapid restoration to the ideal pressure if efficiency is to bemaintained.

Thus, there is a need for a rapid pressure reapply rate during a firststage of brake force restoration, but a slower rate thereafter.

It is known to achieve this, in the context of a flow-valve system, bypulsing the pressure dump solenoid at intervals to cause a repeatedinterruption to the otherwise smooth reapply phase, generating asaw-tooth profile with a flatter overall rise rate. Thus, the emphasisis upon the macro effect of the pulses upon the pressure-rise rate. Inthe prior art, both the intervals and the timing of the first pulse arepredetermined, e.g. by a prescribed relationship with events (e.g.pressure dump and/or reapply duration) from the previous cycle. However,the correlation between these parameters and optimum pulse timing isoften non-linear, rendering the predictions reliable only within anarrow range of operating conditions.

In accordance with the present invention, the pulse timing is controlledby the occurrence of wheel angular-deceleration being equal to or inexcess of a predetermined threshold value, the magnitude of which isadapted in dependence upon events from the previous cycle.

Thus, the pulse timing is arranged to be dependent upon current dataabout the ability of the wheel to resist the applied braking force. Eachpulse thereby assumes increased importance since only through its actionin preventing the wheel from approaching the brink of locking can thewheel's time in the peak adhesion zone be extended.

The main advantage of this principle is that the timing of the pulses ismore reliable, with less likelihood that they will be introduced tooearly, which would waste the flatter section of the pressure rise at arelatively low pressure region. This error could then be compounded bythe effects of coping with the normal pressure rise rate when the skidpressure is reached. (The duration of the flatter re-apply rate has tobe limited to ensure an adequate reaction to unexpected large-scaleimprovement in surface adhesion).

In addition, the provision of the invention will allow the ABS to copemore effectively with naturally occurring cycle-to-cycle variations inoptimum pressure, such as may be caused by surface inconsistency orchanges to wheel loading during manoeuvres.

In some embodiments, the threshold adaption is arranged to be based uponthe proximity of the pulses to the immediately following pressure dump.In other embodiments the threshold adaption is arranged to depend uponthe level of slip which occurred during the previous cycle. Both ofthese have been found to provide effective control of pulse timing.

The principle can be applied to systems featuring fixed orifices or ahold solenoid in place of the flow-valve. In the latter case, thedeceleration threshold controls the timing and/or duration of the holdsolenoid pulses.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 shows a series of operational curves illustrating one example ofa system operating in accordance with the present invention;

FIG. 2 shows how the coefficients of lateral force (CS) and brakingforce (CB) vary with brake slip in a typical example;

FIG. 3 is a flow diagram illustrating the operation of a systemembodying the present invention;

FIGS. 4, 5 and 6 show operational curves used in illustrating theoperation of an alternative system in accordance with the invention;

FIG. 7 illustrates one embodiment in accordance with the presentinvention;

FIG. 8 illustrates a second embodiment in accordance with the presentinvention;

FIG. 9 illustrates a deceleration filter arrangement;

FIG. 10 is a flow diagram illustrating the operation of the decelerationfilter arrangement; and

FIG. 11 shows a series of operational curves relating to the operationof the embodiment of FIG. 8.

FIG. 1 shows the brake pressure (P_(FR)), rotational speed (V_(FR))(scaled in equivalent linear speed), angular acceleration (a_(FR))scaled in equivalent linear acceleration) and ABS dump solenoid status(FR) of a braked wheel running on snow.

The brakes are applied abruptly at a time of 350 ms along the time scaleso that the surface adhesion is overcome almost immediately and thewheel decelerates (-ve acceleration) rapidly, causing ABS intervention.Once the pressure P_(FR) has been relieved, the wheel speed recoversvery quickly, with a high acceleration peak, and then resonates brieflydue to the compliance of the suspension mountings and of the tyreitself. The resonance causes a deceleration and subsequent accelerationof +/-2 g, after which the activity subsides, with small excursions to+/-0.25 g, but then rises again at 950 ms as the wheel slip begins toincrease.

When the deceleration reaches 1.5 g, a fixed pulse X₁ (in this case of 7ms) is arranged to be issued to the solenoid. At this point the pressureis 48 bar, and the pulse causes a fall to 41 bar, which is sufficient toarrest, and even reverse the slip trend, and the deceleration retreats.Eighty-four ms later, however, the pressure has recovered to 53 bar andthe deceleration breaches the -1.5 g level again, causing a further 7 mspulse X₂ which serves to reduce the deceleration and maintain thegradual rate of slip increase. Thereafter no further pulses are allowedso that matters are allowed to take their natural course when thedeceleration increases once more, and a normal, i.e.closed-loop-controlled dump follows when the slip level begins toincrease sharply with the pressure at 56 bar.

The fixed 7 ms pulses X₁ and X₂ have served to prolong the time whichthe wheel is able to spend in the optimum slip range. In this case theslip between the first pulse and the closed-loop dump has ranged from18% to 25%, i.e. on the positive flank of the mu slip curve peak (FIG.2--CB).

As a byproduct of the pulse timing, the overall pressure rise rate hasbeen flattened from the first pulse X₁ onwards.

When the system utilises the first alternative mentioned hereinbefore,wherein the threshold adaption is based upon the proxlimity of thepulses to the immediately following pressure dump, then from the pointwhere the normal dump signal is initiated, the proximity of the pulses Xto the dump is evaluated, and a decision is made whether to adapt the(-1.5 g) threshold to a higher or lower level. In the case illustratedin FIG. 1 the timing was judged to be satisfactory, and the thresholdremains unaltered. In the following cycle, however, after pulse X₃ at1.5 g the wheel skids after a further 50 ms at a lower pressure of 44bar, due possibly to a change in adhesion or wheel loading. The intervalbetween X₃ and the following dump is much shorter than that of thepreceding cycle but the system in accordance with the invention managedpulse X₃ which was able to effect a small delay to the progress of thewheel over the peak of the μ-slip curve.

This time the pulse proximity was adjudged too close, and so thethreshold is incremented to -1.25 g, but the skid pressure has reducedfurther (41 bar) in next cycle so that once again only a single pulse(X₄) has been possible, and the proximity is worse in the precedingcycle.

A further increment of the threshold is made and, with no further fallin skid pressure, the pulse position at the next cycle is once againadjudged to be correct with pulses X₅ and X₆ now being initiated at the-1.0 g threshold.

FIG. 3 illustrates by way of a flow diagram the operation of oneembodiment in accordance with this invention. The various boxes in FIG.3 are identified as follows:

500--Deceleration Dependent Adaption (DDA) pulse

502--Is ABS Active on this wheel?

504--Is time since last real dump<DDA INHIBIT TIME?

506--Is time since last real dump>DDA DISABLE TIME?

508--Have any DDA pulses been issued during the current ABS cycle?

510--Has the maximum number of DDA pulses been issued in this ABS cycle?

512--Is current wheel acceleration more negative than the current DDAthreshold?

514--Issue a DDA pulse to this wheel's solenoid

518--Check for DDA inhibit conditions

516--Check for DDA requirement

520--Adapt the DDA threshold

522--Has a new REAL dump just been demanded

524--Was first DDA pulse in the last ABS cycle too early?

526--Make the DDA threshold more negative

528--Was first DDA pulse in the last ABS cycle too late?

530--Make the DDA threshold more positive

532--Reset DDA timers and counters for this wheel

534--Initialise DDA system for this wheel

536--Repeat for each wheel.

There now follows an explanation of a system which utilises the secondalternative mentioned hereinbefore, wherein the threshold adaption isarranged to depend upon the level of slip which occurred during theprevious cycle, the latter evaluation being used to increment either theactual pressure dump threshold or an associated pulse threshold so as tomaintain the peak slip at an optimum level.

With reference first to FIG. 4, there is illustrated a typical set ofoperational curves for an ABS system from which the pressure dump pointis established, in accordance with well-known practice, as follows.

FIG. 4 shows four traces which are, from top to bottom, wheelacceleration/deceleration a_(FR) (with deceleration threshold value (-b)superimposed upon it), wheel speed V_(FR) (with the vehicle referencespeed), wheel brake pressure P_(FR), and slip threshold ST in terms ofwheel velocity (i.e. kph). On the application of the brake, pressureP_(FR) rises within the brake from (0) to point (E). This causes thebraked wheel to slow down with respect to the vehicle speed over groundfrom (0) towards point (B). The change in velocity of the braked wheelis reflected as a deceleration from (0) towards (A). When thisdeceleration value reaches the deceleration threshold at (A), usuallyset at 1.5 g, the braked wheel velocity (B) is stored. The braked wheelcontinues to slow down and the ABS system dumps the brake pressure whenthe additional speed drop (C) is equal to the slip threshold (D). Thebrake pressure is relieved and the braked wheel speed begins to recoverafter reaching a minimum velocity which, when compared to the vehiclevelocity, provides a peak slip value (F).

The latter combination of deceleration threshold and slip threshold fordetermining the pressure dump (release) point is a standard techniqueand is well known in the field of ABS systems.

In accordance with the second alternative of the present invention, useis made of the peak value (F) of the slip from the previous cycle toalter the position of point (A) (i.e. the deceleration threshold), andtherefore the position of an associated pulse threshold which may bedisplaced from the deceleration threshold by a small increment, for thenext cycle so that when that pulse threshold is met in the followingpressure application the system applies a pulse of short duration in anattempt to arrest the rapid deceleration of the braked wheel associatedwith the loss of adhesion as the wheel passes over the peak of the μslip curve C_(B) of FIG. 2. The `small` increment is preferably positivewith respect to the deceleration threshold but it may also be slightlynegative with respect to said threshold as long as the pulse thresholdlevel is calculated so that pulse firings are triggered prior to any 13real dump firings.

By way of example, FIG. 4 illustrates one aspect of the presentinvention wherein an associated pulse threshold is used in conjunctionwith the deceleration threshold to determine the point at which theadapted pulse signals are applied. The pulse threshold as indicated isfor this example 0.25 g more positive than the deceleration threshold.

Referring now back to FIG. 4, after the cessation of the first wheelskid cycle, the peak slip value (F) is compared against an upper andlower limit of slip to ascertain whether or not the ABS system managedto constrain the braked wheel within or around the optimum peak on the μslip curve of FIG. 2 curve C_(B).

If the peak slip value were to be within the prescribed range of perhaps10% to 20% slip, where slip is defined as the ratio of depression inwheel speed to vehicle speed, then a reasonable level of wheel controlwould be adjudged to have been achieved.

If the peak slip value achieved were less than 10% then this wouldindicate that the skid correction happened too early and optimumutilisation of the available tyre to road adhesion was not obtained. Thesystem would compensate for this by increasing the decelerationthreshold by a small increment of usually 0.25 g, making thedeceleration threshold less sensitive. A similar increment would also beapplied to the pulse threshold which would maintain the 0.25 gdifferential between itself and the deceleration threshold. If the peakslip value were in excess of the upper limit of 20% then this wouldindicate that the skid correction happened too late. The decelerationthreshold would then be lowered by a small increment of 0.25 g in adirection to make it more sensitive. An equivalent increment would alsobe applied to the pulse threshold.

In this particular example it is assumed that the peak slip values aremaintained within the prescribed `window` of upper and lower limits.Upon recovery of the wheel speed, the system reapplies pressure to thebrake on the controlled wheel from point (G) towards point (H). This isaccompanied by a deceleration in the braked wheel which, as the pressureapproaches point (H), brings the deceleration level up to that of thepulse threshold. At this point, a short 7 ms pulse is applied to thedump solenoid of the flow valve modulator. This has the effect ofholding back the progressive increase in deceleration of the wheel.After the short pulse, the pressure resumes its increase towards point(I) which generates another additional deceleration demand causing thewheel deceleration to once more reach the pulse threshold. Another 7 mspulse is applied which again holds off the rapidly increasingprogression of the deceleration of the wheel until finally the wheel,with increasingly applied brake pressure, passes the pulse threshold andreaches the deceleration threshold at (K). After a further increase inslip, equal to the slip threshold, the ABS system applies a `real` dumpof the wheel brake pressure at point (L). The system continues tooperate from cycle to cycle, issuing pulses whenever the pulse thresholdis achieved up to a pre-set maximum number of pulses which, in thiscase, is 2 pulses. Additional pulses could be allowed if so desired.

The present embodiment where a fixed offset from the decelerationthreshold is used for the pulse threshold provides a saving on memory(RAM) in the process controller when compared to a system where thepulse threshold has to be calculated.

With reference to FIG. 5, the pulse threshold and the decelerationthreshold are treated as a common threshold which is incremented inaccordance with the invention. The description of FIG. 5 also detailsthe `adaptive` effect of the invention where both positive and negativeincrements are applied to the pulse/deceleration threshold in dependenceupon the peak value of slip achieved in the previous skid cycle. It mustbe understood that the adaptation as described in the followingdescription could equally be applied to the base system as illustratedin FIG. 4 and hereinbefore discussed.

Referring now to the traces of FIG. 5, the wheel brake pressure isapplied from point E towards point K. This causes the wheel to slow downwith respect to the vehicle, which again is interpreted as a wheeldeceleration. When this deceleration reaches the 1.5 g threshold, thewheel speed is stored and monitored. When the wheel speed decreasesfurther by an amount equal to the slip threshold, then the ABS systemdumps the brake pressure which in turn allows the braked wheel to speedup. The point at which the vehicle's wheel is slowest with respect tothe vehicle itself is, of course, the peak slip value (80 ₁) for thatparticular ABS cycle. In accordance with the present system, this slipvalue is arranged to be compared with an upper and lower limit of, forexample, 20% maximum and 10% minimum. If the measured slip value iswithin this range of slip, i.e. 10% <λ₁ /X₁, >20%, then no adjustment ismade to the deceleration threshold. However, if λ₁ is less than 10% thenthe deceleration threshold is made less sensitive, i.e. more negative,by an increment of, for example, 0.25 g, so that the new decelerationthreshold would become 1.75 g. On the other hand, if λ₁ is greater than20% then the skid cycle was obviously too deep and the dump point needsto occur sooner on the next cycle, i.e. the deceleration threshold ismade more sensitive, i.e. more positive, by an increment of, forexample, 0.25 g to 1.25 g from 1.5 g.

In the illustrated example, it has been assumed that, λ₁ <10% so thatthe deceleration threshold is incremented by -0.25 g at point T₁sometime immediately following the end of the preceding skid cycle. Thewheel speed recovers as the pressure is dumped from point K to L and theABS system recovers control by reapplying the brake from point L towardspoint M. As the pressure increases towards the previous skid pressurethe wheel again begins to slow down with respect to the vehicle, whichis shown as a deceleration at W. When the deceleration level reaches thenew, less sensitive, threshold at -1.75 g, a short (e.g. 7 ms) pulse isapplied to the dump solenoid of the ABS modulator (this could also be a7 ms hold on the inlet solenoid of a 2 solenoid per channel modulatorwhere pulsing of the inlet solenoid controls the rate at which the brakeis reapplied). This pulse causes a short fall or hold in the brakepressure rise rate which is reflected as a short halting of the increasein deceleration Y. This short holding back of the skid progressionactually produces a much improved adhesion utilisation as the tyre toroad slip value is held at a point that is very close to the peak on theμ slip curve CB of FIG. 2 for an extended period.

If it is now assumed that λ₂ was actually in excess of 20% on thissecond cycle, then an associated incremental step of 0.25 g would beapplied at T₂ to the present -1.75 g threshold. This would haveindicated that the second skid cycle was too deep and the next cycleneeds to be initiated at a slightly earlier point. The brake pressure isnow reapplied from point P towards point S. At point Q the wheeldeceleration has reached the new deceleration threshold of 1.5 g and ashort 7 ms pulse is applied to relieve and recover the increasingdeceleration. At point R the wheel deceleration again reaches the 1.5 gdeceleration threshold and another, second, 7 ms pulse is applied whichhas a similar effect of recovering, for a short period, the onset ofslip. Thereafter, in this particular instance, no further pulses areallowed and the wheel deceleration is allowed to increase normally to apoint where the ABS system intervenes to dump the brake pressure at S.Depending upon the system, additional pulses may be allowed, but forthis example two pulses were enough to extend the period in which thewheel remained in the peak slip area on the μ slip curve of FIG. 2. Thisperiod is indicated at Z. The system continues in adjusting thedeceleration threshold in accordance with the peak slip values from theprevious cycle, i.e.λ₃ etc.

A situation could occur in practice where the effective slowing down inpressure rise rate is not preferred. In particular, this could happenwhen the vehicle passes from a low mu surface to a high mu surface. Onthe low mu surface, a relatively low pressure is applied to produce askid cycle whereas on a high mu surface a relatively high pressure isrequired. At the transition from low to high, the build-up in a pressureneeds to happen as quickly as possible in order to utilise the adhesionon the improved surface.

Referring now to FIG. 6, a system, as described substantially ashereinbefore where both pulse and deceleration thresholds are used withthe pulse threshold being displaced from the deceleration threshold by asmall positive increment of 0.25 g, is shown working initially on a lowmu surface with relatively low skid pressures passes at someintermediate transition point as illustrated at (T), from the low musurface onto a surface of a higher friction co-efficient. At this pointit is assumed that the previous skid cycles had caused the pulse anddeceleration thresholds to adapt towards the more sensitive region sothat at the transition point (T) the deceleration threshold is set to1.0 g as shown at (B) and the pulse threshold is offset a further 0.25 gin the positive direction at -0.75 g as shown at (A). At this point theapplied braking pressure is too small to generate a wheel decelerationwhich would cause a `real` skid, but as the pressure increases there isa tendency for the pressure to generate a sufficiently largedeceleration to cause a pulse firing should the pulse threshold remainat the -0.75 g level. A pulse firing at this stage would, of course,delay the required rapid increase in pressure to suit the improved roadconditions. This can be prevented by incrementing the pulse threshold(with respect to the deceleration threshold) at suitable time intervals,e.g. 150 ms. After each 150 ms period as shown at X, Y and Z, the pulsethreshold is incremented a further -0.25 g thus making the system lesssensitive to pulse demands. Therefore at point (C) the pulse thresholdis -1.0 g, now equal to the deceleration threshold, then after a further150 ms an additional increment is added so that at (E) the pulsethreshold is -1.25 g. A further increment of -0.25 g is applied at theend of the next 150 ms period and from this point onwards the pulsethreshold could either be maintained at the adjusted level or optionallythe 150 ms increment could continue until the following real dumpfiring.

In addition to the 150 ms increments to the pulse threshold a resettingof the deceleration threshold, as illustrated, could take place at the450 ms point. The choice of 450 ms is based on an assumption of when,after a real brake dump it would be safe to assume that, in the absenceof any further real dump firings, that the tyre to road frictionco-efficient had improved. Therefore it must be understood that whilstthis specific time period is applied to this example, the actual timeperiod may vary in dependence upon the specific installation. Uponexpiry of the reset period the deceleration threshold is reset to itsbase level of 1.5 g. This then ensures that the next skid cycle, nowadjudged to be on a much improved surface, utilises the availableadhesion. Had the deceleration threshold not been reset, then the nextreal dump would have fired prematurely, with a shallow skid cycle,having dumped too early from a lower deceleration level.

FIG. 7 shows one possible embodiment of an embodiment of the first typedescribed hereinbefore wherein the threshold adaption is arranged to bebased upon the proximity of the pulses to the immediately followingpressure dump.

Reference number 100 identifies a conventional wheel speed sensorassociated with one of the vehicle wheels (not shown). The output signalfrom the sensor 100 is conditioned at 102 and passed both to a wheelspeed calculating unit 104 and a wheel deceleration calculating unit106. The resulting calculated wheel speed and wheel deceleration signalsare stored in speed and deceleration stores 108, 110. The vehicle speedis calculated in a speed calculation unit 112 and stored in a vehiclespeed store 114. The wheel speed signal from store 108 is subtracted inan arithmetic unit 116 from the vehicle speed held in the store 114 toprovide a signal (SLIP) representative of the slip of that wheelrelative to the road surface. The SLIP signal is applied to one input(A) of a comparator 118 where it is compared with a reference signal (B)corresponding to a predetermined slip level, in this example of 1 kph,provided by a store 119. If the result of the comparison shows that A≧Bthen a signal is applied to one input of an AND gate 120.

Between the units 106 and 110 there can optionally be located adeceleration filter 109 as described further hereinafter in connectionFIGS. 9 and 10.

The output of the AND gate 120 is coupled via an element 122 to theinput of a solenoid driver 124 which provides a signal on a line 126leading to a solenoid (not shown) whose actuation results in the fluidpressure to the relevant brake actuator being released (dumped). Theelement 122 is arranged to emit a short duration pulse to the solenoiddriver 124 when it receives the output signal of the AND gate 120.

Also connected to the input of the solenoid driver 124 is the output ofa standard ABS detection system shown collectively by the box 128. Thissystem 128 actuates the solenoid driver 124 to cause closed loopcontrolled dumping of the actuating pressure in accordance withconventional ABS practice and is not illustrated in detail.

The dump signal on the line 126 provided by the solenoid driver 124 isalso fed by a line 130 to the inputs of a "start of closed loopcontrolled dump" unit 132 and an "end of closed loop controlled dump"unit 134. The output of the unit 132 is connected firstly to a "cleartimer since end of last closed loop controlled dump" unit 136, secondlyto first inputs of two AND gates 138,140, thirdly to the input of a"clear pulse count" unit 142 and fourthly to the input of an inverter144. The output of the inverter 144 is applied to one input of an ANDgate 146 whose other input is connected to the output of a comparator148, having one input connected to a zero reference voltage and a secondinput connected to an output of a "first APT (adaptive pulse threshold)pulse timer store" 150. The output of the AND gate is connected to thefirst APT store 150 via a unit 152 which is adapted to increment thefirst APT once per second. An element marked 154 provides a signal whenthe main ABS is "not active" and passes this signal to one input of aunit 156 which acts to clear the first APT store 150 when activated. Theunit 156 also receives the output of a NAND gate 158 whose three inputsare connected respectively to the output of an OR gate 160 and theoutputs of the AND gates 138 and 140. The outputs of the AND gate 140 isalso connected to the input of an "increment" unit 162 coupled to oneinput of an "APT threshold store"0 164 connected to one input of acomparator 166. The other input of the comparator 166 is connected tothe wheel deceleration store 110 and its output is connected to anotherinput of the AND gate 120. A still further input of the AND gate 120 isconnected to a unit 168 which produces an output whenever the main ABS(128) is active. The first APT pulse timer store 150 also providessignals to the B inputs or two further comparators 170,172. The A inputof comparator 170 is connected to a "lower limit" reference 174 and theA input of the comparator 172 is connected to an "upper limit" reference176. Comparators 170 and 172 provide outputs when A<B and A>B at theirrespective inputs.

The "clear pulse count" unit 142 is connected to a pulse count store 178which receives pulse count signals from the output of the AND gate 120via an "increment pulse count" unit 180. The output of the store 178 isconnected to one input of a comparator 182 where it is compared to areference level of "1". If the output of comparator shows that the countis one, then a signal is provided to a "start first APT pulse timer"element 184, coupled to the first APT pulse timer store 150.

The output of the "clear timer" unit 136 is connected to a timer (PaBrake) 186 which also receives an input from the "end of closed loopcontrolled dump" unit 134, via a "start timer since end of last closedloop dump" element 188. The output of the timer 186 is connected to oneinput (B) of a comparator 190 whose other input (A) receives a referencevalue, eg. 150 ms. The output (X) of the comparator 190 is fed to oneinput of a "decrement unit" 192, having a second input connected to theoutput of the AND gate 138. The output of the "decrement" unit 192 leadsto a second input of the APT threshold store 164.

In the event that the deceleration filter 109 of FIG. 7 is used, theroutine of FIG. 10 can be applied.

The various boxes in FIG. 10 are identified as follows:

260--Deceleration filter

262--Calculate new deceleration

263--Calculate DIFFERENCE, ie new deceleration minus old deceleration

264--Is magnitude of DIFFERENCE ≦6?

267--Is direction of change up?

265--Mark direction of change NONE for next time

269--Mark direction of change UP for next time

269--Mark direction of change DOWN for next time

270--Is direction of change same as last time?

271--Limit new deceleration to "old deceleration" or to + or -6

266--- Save new deceleration

261--- Done

FIG. 10 shows a deceleration filter routine also performed by anelectronic control unit, which recycles between the entry 260 and exit261 of the routine. At step 262, the new deceleration is calculated fromthe signals provided by the wheel sensor. At step 263, the differencebetween the new deceleration and the old deceleration i.e. thatcalculated during the preceding performance of the routine, iscalculated and, at step 264, compared with a predetermined differenceindicated in FIG. 10 as "6". If the difference is less than 6, step 265notes that there is no directional change for the succeeding cycle ofthe routine and, at step 266, the calculated value of the newdeceleration is saved for the next cycle. Step 267 determines whetherthe direction of change in the difference is up or down and steps 268and 269 mark the direction as up or down, respectively, for the nextcycle. Step 270 checks whether the direction of change is the same asduring the preceding cycle and, if so, passes control to the step 266.If not, step 271 limits the new deceleration to the old decelerationplus or minus 6 and the limited value is saved in the step 266.

FIG. 9 shows an example of hardware for achieving the routine of FIG.10. The signal conditioning unit 201 corresponds to the signalconditioning unit 102 of FIG. 7. Deceleration calculation is carried outat 202 and fed to a + input of an arithmetic unit 204. The - input ofthe arithmetic unit 203 is coupled to a store 217 holding the "olddeceleration" . Store 6 is coupled to the output of a "new deceleration"store 216 whose two inputs are connected respectively to "allow" units214 and 215. "Allow" unit 214 receives the output of the arithmetic unit205 and an inverter 212 and "allow" unit 215 receives the output of thedeceleration calculation unit 202 and the output of a comparator 211.The + input of the comparator receives a "direction of change" signalfrom a unit 207 and its - input receives a signal from an "olddirection" unit 210. The output of the comparator is also applied to theinput of the inverter 212. The unit 217 receives its input from anelement 209 which determines the direction of change of the decelerationfrom the output of the arithmetic unit 204 and the output of a furthercomparator 203 whose inputs are the arithmetic unit output and areference signal provided by a unit 206 corresponding to the permissibleslow limit (corresponding to the value "6" in the flow diagram of FIG.10). The reference slow limit from unit 206 is added in a furtherarithmetic unit 205 to the output of the "old decelerator" unit 217 toprovide an input to "allow" unit 214.

The objective of the apparatus of FIG. 9 is to provide a reliable valueof wheel deceleration by filtering out any variations due to wheelsuspension movement. This is achieved by first checking if the currentlycalculated value of deceleration is within a predetermined "slew" limit.If it is, then this calculation is allowed to update the stored value ofdeceleration. If the calculation is in excess of the "slew limit" value,then a check is made to see if deceleration calculated is in the samedirection as that calculated in the previous processor cycle. If it isthen it is assumed that the deceleration calculation is in order and thewheel is seeing a true deceleration which happens to be in excess of thepredetermined "slew limit" . This calculated deceleration is then usedto update the deceleration value. If the direction of change of thedeceleration is different to that seen in the previous processor cycle,then it is assumed that the calculated deceleration is in error and isnot used to update the deceleration value. Instead, an assumed newdeceleration is produced which is the sum of the old deceleration plusor minus the "slew limit" value. The "slew limit" value is added if theprevious direction of change was positive and subtracted if the previousdirection of change was negative i.e. if the previous indication wasthat the wheel was increasing in deceleration then the "slew" limit isadded to produce a further assumed increase etc.

The signal conditioning of the wheel speed signal is provided by asignal conditioning unit 201. The values produced are passed to adeceleration calculator 202 which produces a current deceleration value.This current value is fed forward to an allow switch 214 which isenabled by the following processes.

The current deceleration has subtracted from it the value of olddeceleration 217, the old deceleration having been calculated in theprevious processor cycle. This deceleration error value produced inadder 204 provides one input to a direction of change calculating device209. The other input to the direction of change device 209 is the valueof deceleration error minus a predetermined slew limit 206. These twovalues are used in accordance with the processes shown in the flow chartof FIG. 10, marked as boxes 265, 268, 269 to produce a direction ofchange signal which is stored in a direction of change store 207. If themagnitude of the difference between the current and old decelerationvalues is less than or equal to the slew limit then the direction ofchange is marked as "NONE" and stored as NONE in store 207. If thedeceleration error signal is positive i.e. UP, then the direction ofchange is stored as UP. Likewise if the direction of change is DOWN thenthe direction of change is stored as DOWN. The current direction ofchange signal is fed to the one input of each of two comparators. Afirst comparator 211 compares the new direction of change stored in 207with the previous processor cycle direction of change held in store 210.If the direction of change is the same i.e. both UP or both DOWN thenthe input to the allow switch goes high and the calculated deceleration202 is stored as the new deceleration in store 216. Likewise, thedirection of change signal from store 207 is compared in comparator 218with a state NONE from store 219. Thus, if the stored change indirection is NONE, i.e. the calculated deceleration is less than theslew limit, the output of the comparator 218 goes high thereforeenabling the allow switch which in turn permits the storage of thecalculated deceleration 202 as the new deceleration in store 216. Shouldeither of the two comparators go high, then the inverter 212 switchesthe input of the second allow switch 214 off and likewise, shouldneither of the comparators go high then the first allow switch 215 isdisabled and the inverter 212 enables the second allow switch 214.Enabling this second allow switch loads the new deceleration store withthe assumed change in deceleration composed of the slew limit valueobtained from store 206 corrected for sign by correction unit 220 whichchanges the sign of the slew limit value in accordance with thedirection of change of deceleration as stored in store 207. Thiscorrected slew limit value is added to the old value of deceleration asstored in store 217 by the adder 205 and is therefore loaded into thenew deceleration store 216 when the second allow switch is enabled. Thenew deceleration stored in store 216 is provided for use in the ensuingABS routine. once a new value is stored in the store 216, the value ispassed to the old deceleration store 217 for updating the olddeceleration value for use in the next processor cycle. During thisupdate a load change device is enabled which permits the old directionof change store 210 to be updated with the new direction of change valuefrom the direction of change store 107.

Therefore, should the calculated value of deceleration be withinpredetermined limits then this value is used as the wheel decelerationsignal for the remainder of the ABS routine, during that processorcycle. If the calculated deceleration value is outside the predeterminedlimits then an assumed value of deceleration is used as above.

Returning now to FIG. 7, the operation of the apparatus is described inconnection with the diagrams of FIG. 1.

At FIG. 1, point 1, the vehicle brakes are applied. At this point, theABS is not active and therefore no short duration pulses are issued atpoint 2 when the wheel deceleration traverses the APT threshold.

At point 3 in FIG. 1, a closed loop controlled dump is issued via thestandard ABS detection mechanism 128. This event clears the timer136,186 (pa break) and also clears the APT pulse counter 178,180. As aconsequence of point 3, the ABS is now ACTIVE on the wheel underconsideration.

At point 4, the closed loop controlled dump ends, starting the timer186.

At point 5, the wheel deceleration exceeds the APT threshold in 164 and,since in this particular example, the optional slip check was notincorporated, a short duration pulse is issued by the pulse generationunit 122 at point 6.

The latter event triggers "increment pulse count" 180 which in turncauses the output of comparator 182 to be " ", thereby starting the"first APT timer" 184. This causes the output of comparator 148 to be "", thereby enabling the timer periodic increment at unit 152.

At point 8, a further short duration pulse is issued by the pulsegenerator 122 since the wheel deceleration once more crosses the APTthreshold at point 7.

At point 9, a second closed loop controlled dump is issued via themechanism 128. At this event, the outputs of comparators 172 and 170 areswitched through to the APT threshold modifiers 192 and 162,respectively.

The effect of this is to increment (make more sensitive) the APTthreshold if the time measured by the "first APT pulse timer" 150 islonger than the UPPER LIMIT 176 or to maintain the APT threshold at itscurrent value if the time is between these limits. After the APTthreshold has been adjusted (or not as the case may be) the "first APTpulse timer" 150 is cleared by the combination of gates terminating atgate 158 and the "clear first APT" unit 156.

The latter operation repeats continuously.

There now follows, with reference to FIG. 8, a designation of anembodiment of a deceleration dependent adaptive pulse generator inaccordance with the present invention but of the second type in whichthe threshold adaption depends on the level of slip which occurredduring the previous ABS cycle. Those parts which are identical to theembodiment of FIG. 7 are given the same reference numbers.

The "start of closed loop controlled dump" unit 132, which as beforereceives a signal from the line 126 when the ABS is activated, isconnected to the Pa brake timer 186 via the clear timer unit 136.Likewise the "end of closed loop controlled dump" unit 134 is connectedto the timer 186 via the "start timer since end of last closed loopdump" element 188. The timer 186 output is applied to the (A) input ofthe comparator 190, the (B) input of which is connected to referencevalue, eg 150 ms. The output of comparator 190 forms one input of thedecrement element 192, which also receives an input from an AND gate400. The decrement element 192 is coupled to one input of the APTthreshold unit 164 whose output is coupled to one input (B) of thecomparator 166. The other input of the APT threshold unit 164 isconnected to the output of an AND gate 402 via the incrementing element162. One input of the AND gate 402 is connected to one input of the ANDgate 400 and also to a "start of new skid detection sequence" element404. The other inputs of the AND gates 400 and 402 are connected to theoutput of further comparators 406,408, respectively. The one (A) inputsof the comparators 406,408 are connected together and also to the outputof a "peak slip" store 410, whilst the other (B) inputs of thecomparators 406,408 are connected respectively to an "upper limit"reference 412 and a "lower limit" reference 414. The input of the "peakslip" store 410 is connected via a "track and hold" unit 415 to theoutput of the arithmetic unit 116 carrying a signal representative ofthe prevailing wheel slip.

The AND gate 120 has a further input connected to the output of acomparator 416 whose one (A) input is connected to an element 418 whichenables the "maximum number of pulses" to be preset, and whose other (B)input is connected to a pulse count store 420. The latter store iscoupled to the output of the AND gate 120 via an "increment pulse count"element 422.

The peak slip store 410 also receives a reset input from a reset element133 connected to the "start of closed loop controlled dump" unit 132.AND gate 120 receives a further input from an "ABS active" element 121.The AND gates 400, 402 receive a further input from an "ABS active"element 391, whose output is also connected via an inverter 395 andinitialiser 393 to the APT threshold unit 164.

The operation of the embodiment of FIG. 8 is now described withreference to the operational curves of FIG. 11.

In the example, the upper and lower slip thresholds used in the adaptionare -5% and -10%, respectively, compared to the embodiments describedhereinbefore which use -10% and -20%.

(i) An ABS cycle begins at time 2.394 secs in FIG. 11, see point A inFIG. 11. The main solenoid, controlled via line 126, receives multiple"closed loop controlled dumps" (CLCDs) during this cycle but, since thewheel is in slip from point A to point B, these firings are in factconsidered as one. (it is well known that a CLCD does not have to becontinuous). Thus, the start of the ABS cycle at point A triggers the"Clear timer unit" 136 and the reset element 133, clearing the "timesince last CLCD" timer 136 and the peak slip store 410, respectively.

(ii) From time A, the peak slip value is tracked by track and hold unit415 and the peak value is stored in store 410. In this example, peakslip occurs at point C in FIG. 11 and is more negative than the lowerthreshold limit (-10%).

(iii) At point D, a new skid detection sequence is started and so ANDgates 402,400 are enabled, switching the results from comparators408,406 through to the increment/decrement mechanisms at elements 162,192. In this case, because the peak slip was below the lower limit, theAPT threshold at element 164 is incremented to make the system moresensitive on the next ABS cycle.

(iv) At points E and F in FIG. 11, the wheel deceleration held atelement 110 is more negative than or equal to the APT threshold atelement 166 and the (optional) slip qualification is met at element 118and so a short duration pulse (APT) is issued. At point G in FIG. 11, afurther APT is issued by the same criteria but this is followedimmediately by the start of a new CLCD at point H.

(v) At point H, the elements 136 and 133 are triggered again and theprocess described in (ii), (iii) and (iv) above begins again.

(vi) Point J is the start of a new CLCD. The processes in (ii), (iii)and (iv) take place but this time the peak slip value is more positivethan the upper limit (point K, in FIG. 11). Thus process (iii) in thiscase causes the APT threshold to be decremented to make the system lesssensitive for the next ABS cycle.

We claim:
 1. An anti-skid braking system for wheeled vehicles havingfluid actuated brakes associated with the vehicle wheels,comprising:speed sensors associated with the vehicle wheels, a controldevice responsive to speed signals from the speed sensors to actuate apressure dump device to periodically release the fluid pressure appliedto the brake of any wheel which is determined to be about to lock and tolater re-apply the actuating pressure to that brake when the tendency ofthat wheel to lock has reduced, a pressure re-apply phase being presentfrom a wheel recovery point after a full pressure dump until reaching athreshold for starting a full pressure dump, means for pulsing saidpressure dump device at intervals with at least one fixed duration pulseduring the pressure re-apply phase to cause interruption to there-application of pressure to that brake, means for controlling theinitiation of said at least one fixed length pulse in response to thewheel angular deceleration being above a predetermined threshold value,and means for adjusting the magnitude of said predetermined thresholdvalue in dependence upon information from the previous anti-lock brakingcycle.
 2. An anti-skid braking system as claimed in claim 1, wherein thethreshold adjustment for the current skid cycle is based on theproximity of a pressure pulse in the preceding cycle to an immediatelypreceding pressure dump.
 3. An anti-skid braking system as claimed inclaim 2, including a deceleration filter which receives decelerationvalues calculated from the signals provided by said wheel speed sensorsand which comprises checking means for checking the currently calculatedvalue of deceleration against a predetermined limit and wherein if thecalculated deceleration is less than said predetermined limit, thecalculated value is used to update a stored value of deceleration.
 4. Ananti-skid braking system as claimed in claim 1, wherein the thresholdadjustment is dependent upon the measured level of wheel slip whichoccurred during the previous anti-lock braking cycle.
 5. An anti-skidbraking system as claimed in claim 4, including a deceleration filterwhich receives deceleration values calculated from the signals providedby said wheel speed sensors and which comprises checking means forchecking the currently calculated value of deceleration against apredetermined limit and wherein if the calculated deceleration is lessthan said predetermined limit, the calculated value is used to update astored value of deceleration.
 6. An anti-skid braking system as claimedin claim 1, including a deceleration filter which receives decelerationvalues calculated from the signals provided by said wheel speed sensorsand which comprises checking means for checking the currently calculatedvalue of deceleration against a predetermined limit and wherein if thecalculated deceleration is less than said predetermined limit, thecalculated value is used to update a stored value of deceleration.
 7. Ananti-skid braking system as claimed in claim 6, including means which,if the calculated deceleration is in excess of said predetermined limitvalue, establish whether the direction of change of the calculateddeceleration is in the same direction as that calculated in the previouscycle of the control device.
 8. An anti-skid braking system as claimedin claim 7, wherein, if the direction of change in deceleration is inthe same as in the previous cycle, then the deceleration valuecalculated is stored as the new value of deceleration.
 9. An anti-skidbraking system as claimed in claim 8, wherein of the direction of changeif the calculated deceleration is different to the stored from theprevious control device cycle, the old deceleration value and thepredetermine limit value are used to update the new stored decelerationvalue.
 10. An anti-skid braking system as claimed in claim 9, whereinthe predetermined limit value is added to the old stored decelerationvalue if the previous direction of change was positive and subtracted ifthe previous direction of change was negative.
 11. A deceleration filterfor use in an anti-skid braking system for wheeled vehicles, of the typecomprising speed sensors associated with the vehicle wheels, a controldevice responsive to speed signals from the speed sensors to actuate apressure dump device to periodically release the fluid pressure appliedto the brake of any wheel which is determined to be about to lock and tolater re-apply the actuating pressure to that brake when the tendency ofthat wheel to lock has reduced, the deceleration filter being adapted toreceive deceleration values calculated from the signals provided by saidwheel speed sensors and comprising checking means adapted to check thecurrently calculated value of deceleration against a predetermined limitand, if the calculated deceleration is less than said predeterminedlimit, to use the calculated value to update a stored value ofdecelerations.
 12. A deceleration filter as claimed in claim 11including means which, if the calculated deceleration is in excess ofsaid predetermined limit value, establishes whether the direction ofchange of the calculated deceleration is in the same direction as thatcalculated in the previous cycle of the control device.
 13. Adeceleration filter as claimed in claim 12 wherein, if the direction ofchange in the deceleration is the same as in the previous cycle, thenthe deceleration value calculated is stored as the new value ofdeceleration.
 14. A deceleration filter as claimed in claim 12, whereinif the direction of change of the calculated deceleration is differentto that stored from a previous control device cycle, the olddeceleration value and the predetermined limit value are used to updatethe new stored deceleration value.
 15. A deceleration filter as claimedin claim 14, wherein the predetermined limit value is added to the oldstored deceleration value if the previous direction of change waspositive and subtracted if the previous direction of change wasnegative.